Network Working Group M. Zanaty
Internet-Draft E. Berger
Intended status: Standards Track S. Nandakumar
Expires: April 26, 2019 Cisco Systems
October 23, 2018
Frame Marking RTP Header Extensiondraft-ietf-avtext-framemarking-08
Abstract
This document describes a Frame Marking RTP header extension used to
convey information about video frames that is critical for error
recovery and packet forwarding in RTP middleboxes or network nodes.
It is most useful when media is encrypted, and essential when the
middlebox or node has no access to the media decryption keys. It is
also useful for codec-agnostic processing of encrypted or unencrypted
media, while it also supports extensions for codec-specific
information.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 26, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Internet-Draft Frame Marking October 2018
In this document, an "RTP switch" is used as a common short term for
the terms "switching RTP mixer", "source projecting middlebox",
"source forwarding unit/middlebox" and "video switching MCU" as
discussed in [RFC7667].
+---+ +------------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| RTP |
+---+ | Switch | +---+
| C |<---->| |<---->| D |
+---+ +------------+ +---+
Figure 1: RTP switch
In order to properly support switching of video streams, the RTP
switch typically needs some critical information about video frames
in order to start and stop forwarding streams.
o Because of inter-frame dependencies, it should ideally switch
video streams at a point where the first frame from the new
speaker can be decoded by recipients without prior frames, e.g
switch on an intra-frame.
o In many cases, the switch may need to drop frames in order to
realize congestion control techniques, and needs to know which
frames can be dropped with minimal impact to video quality.
o Furthermore, it is highly desirable to do this in a payload
format-agnostic way which is not specific to each different video
codec. Most modern video codecs share common concepts around
frame types and other critical information to make this codec-
agnostic handling possible.
o It is also desirable to be able to do this for SRTP without
requiring the video switch to decrypt the packets. SRTP will
encrypt the RTP payload format contents and consequently this data
is not usable for the switching function without decryption, which
may not even be possible in the case of end-to-end encryption of
private media [I-D.ietf-perc-private-media-framework].
By providing meta-information about the RTP streams outside the
encrypted media payload, an RTP switch can do codec-agnostic
selective forwarding without decrypting the payload. This document
specifies the necessary meta-information in an RTP header extension.
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Internet-Draft Frame Marking October 20182. Key Words for Normative Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Frame Marking RTP Header Extension
This specification uses RTP header extensions as defined in
[RFC8285]. A subset of meta-information from the video stream is
provided as an RTP header extension to allow an RTP switch to do
generic selective forwarding of video streams encoded with
potentially different video codecs.
The Frame Marking RTP header extension is encoded using the one-byte
header or two-byte header as described in [RFC8285]. The one-byte
header format is used for examples in this memo. The two-byte header
format is used when other two-byte header extensions are present in
the same RTP packet, since mixing one-byte and two-byte extensions is
not possible in the same RTP packet.
This extension is only specified for Source (not Redundancy) RTP
Streams [RFC7656] that carry video payloads. It is not specified for
audio payloads, nor is it specified for Redundancy RTP Streams. The
(separate) specifications for Redundancy RTP Streams often include
provisions for recovering any header extensions that were part of the
original source packet. Such provisions SHALL be followed to recover
the Frame Marking RTP header extension of the original source packet.
Source packet frame markings may be useful when generating Redundancy
RTP Streams; for example, the I and D bits can be used to generate
extra or no redundancy, respectively, and redundancy schemes with
source blocks can align source block boundaries with Independent
frame boundaries as marked by the I bit.
A frame, in the context of this specification, is the set of RTP
packets with the same RTP timestamp from a specific RTP
synchronization source (SSRC).
3.1. Short Extension for Non-Scalable Streams
The following RTP header extension is RECOMMENDED for non-scalable
streams. It MAY also be used for scalable streams if the sender has
limited or no information about stream scalability. The ID is
assigned per [RFC8285], and the length is encoded as L=0 which
indicates 1 octet of data.
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Internet-Draft Frame Marking October 2018
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=0 |S|E|I|D|0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following information are extracted from the media payload and
sent in the Frame Marking RTP header extension.
o S: Start of Frame (1 bit) - MUST be 1 in the first packet in a
frame; otherwise MUST be 0.
o E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame;
otherwise MUST be 0. Note that this SHOULD match the RTP header
marker bit when the latter is reliable.
o I: Independent Frame (1 bit) - MUST be 1 for frames that can be
decoded independent of temporally prior frames, e.g. intra-frame,
VPX keyframe, H.264 IDR [RFC6184], H.265 IDR/CRA/BLA/RAP
[RFC7798]; otherwise MUST be 0.
o D: Discardable Frame (1 bit) - MUST be 1 for frames the sender
knows can be discarded, and still provide a decodable media
stream; otherwise MUST be 0.
o The remaining (4 bits) - are reserved for future use for non-
scalable streams; they MUST be set to 0 upon transmission and
ignored upon reception.
3.2. Long Extension for Scalable Streams
The following RTP header extension is RECOMMENDED for scalable
streams. It MAY also be used for non-scalable streams, in which case
TID, LID and TL0PICIDX MUST be 0. The ID is assigned per [RFC8285],
and the length is encoded as L=2 which indicates 3 octets of data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID | LID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following information are extracted from the media payload and
sent in the Frame Marking RTP header extension.
o S: Start of Frame (1 bit) - MUST be 1 in the first packet in a
frame within a layer; otherwise MUST be 0.
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Internet-Draft Frame Marking October 2018
o E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame
within a layer; otherwise MUST be 0. Note that the RTP header
marker bit MAY be used to infer the last packet of the highest
enhancement layer.
o I: Independent Frame (1 bit) - MUST be 1 for frames that can be
decoded independent of temporally prior frames, e.g. intra-frame,
VPX keyframe, H.264 IDR [RFC6184], H.265 IDR/CRA/BLA/RAP
[RFC7798]; otherwise MUST be 0. Note that this bit only signals
temporal independence, so it can be 1 in spatial or quality
enhancement layers that depend on temporally co-located layers but
not temporally prior frames.
o D: Discardable Frame (1 bit) - MUST be 1 for frames the sender
knows can be discarded, and still provide a decodable media
stream; otherwise MUST be 0.
o B: Base Layer Sync (1 bit) - MUST be 1 if the sender knows this
frame only depends on the base temporal layer; otherwise MUST be
0. If no scalability is used, this MUST be 0.
o TID: Temporal ID (3 bits) - The base temporal layer starts with 0,
and increases with 1 for each higher temporal layer/sub-layer. If
no scalability is used, this MUST be 0.
o LID: Layer ID (8 bits) - Identifies the spatial and quality layer
encoded, starting with 0 and increasing with higher fidelity. If
no scalability is used, this MUST be 0 or omitted to reduce
length. When omitted, TL0PICIDX MUST also be omitted.
o TL0PICIDX: Temporal Layer 0 Picture Index (8 bits) - Running index
of base temporal layer 0 frames when TID is 0. When TID is not 0,
this indicates a dependency on the given index. If no scalability
is used, or the running index is unknown, this MUST be omitted to
reduce length. Note that 0 is a valid running index value for
TL0PICIDX.
The layer information contained in TID and LID convey useful aspects
of the layer structure that can be utilized in selective forwarding.
Without further information about the layer structure, these
identifiers can only be used for relative priority of layers. They
convey a layer hierarchy with TID=0 and LID=0 identifying the base
layer. Higher values of TID identify higher temporal layers with
higher frame rates. Higher values of LID identify higher spatial
and/or quality layers with higher resolutions and/or bitrates.
With further information, for example, possible future RTCP SDES
items that convey full layer structure information, it may be
possible to map these TIDs and LIDs to specific frame rates,
resolutions and bitrates. Such additional layer information may be
useful for forwarding decisions in the RTP switch, but is beyond the
scope of this memo. The relative layer information is still useful
for many selective forwarding decisions even without such additional
layer information.
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Internet-Draft Frame Marking October 20183.2.1.4. VP8 LID Mapping
The following shows the header extension for VP8 [RFC7741] that
contains only temporal layer information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=2 | L=2 |S|E|I|D|B| TID |0|0|0|0|0|0|0|0| TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.2.1.5. Future Codec LID Mapping
The RTP payload format specification for future video codecs SHOULD
include a section describing the LID mapping and TID mapping for the
codec. For example, the LID/TID mapping for the VP9 codec is
described in the VP9 RTP Payload Format [I-D.ietf-payload-vp9].
3.3. Signaling Information
The URI for declaring this header extension in an extmap attribute is
"urn:ietf:params:rtp-hdrext:framemarking". It does not contain any
extension attributes.
An example attribute line in SDP:
a=extmap:3 urn:ietf:params:rtp-hdrext:framemarking
3.4. Usage Considerations
The header extension values MUST represent what is already in the RTP
payload.
When an RTP switch needs to discard a received video frame due to
congestion control considerations, it is RECOMMENDED that it
preferably drop frames marked with the D (Discardable) bit set, or
the highest values of TID and LID, which indicate the highest
temporal and spatial/quality enhancement layers, since those
typically have fewer dependenices on them than lower layers.
When an RTP switch wants to forward a new video stream to a receiver,
it is RECOMMENDED to select the new video stream from the first
switching point with the I (Independent) bit set in all spatial
layers and forward the same. An RTP switch can request a media
source to generate a switching point by sending Full Intra Request
(RTCP FIR) as defined in [RFC5104], for example.
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Internet-Draft Frame Marking October 20183.4.1. Relation to Layer Refresh Request (LRR)
Receivers can use the Layer Refresh Request (LRR)
[I-D.ietf-avtext-lrr] RTCP feedback message to upgrade to a higher
layer in scalable encodings. The TID/LID values and formats used in
LRR messages MUST correspond to the same values and formats specified
in Section 3.2.
Because frame marking can only be used with temporally-nested
streams, temporal-layer LRR refreshes are unnecessary for frame-
marked streams. Other refreshes can be detected based on the I bit
being set for the specific spatial layers.
3.4.2. Scalability Structures
The LID and TID information is most useful for fixed scalability
structures, such as nested hierarchical temporal layering structures,
where each temporal layer only references lower temporal layers or
the base temporal layer. The LID and TID information is less useful,
or even not useful at all, for complex, irregular scalability
structures that do not conform to common, fixed patterns of inter-
layer dependencies and referencing structures. Therefore it is
RECOMMENDED to use LID and TID information for RTP switch forwarding
decisions only in the case of temporally nested scalability
structures, and it is NOT RECOMMENDED for other (more complex or
irregular) scalability structures.
4. Security Considerations
In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP
header extensions are authenticated but usually not encrypted. When
header extensions are used some of the payload type information are
exposed and visible to middle boxes. The encrypted media data is not
exposed, so this is not seen as a high risk exposure.
5. Acknowledgements
Many thanks to Bernard Aboba, Jonathan Lennox, and Stephan Wenger for
their inputs.
6. IANA Considerations
This document defines a new extension URI to the RTP Compact
HeaderExtensions sub-registry of the Real-Time Transport Protocol
(RTP) Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:framemarkinginfo
Description: Frame marking information for video streams
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